Polyurethaneurea elastic fiber exhibits, on the basis of its unique elastic properties, an excellent elastic stretch power, a high elongation and high elastic recovery, and has been widely used in versatile application fields such articles of clothing and in products for industrial use. In the field of pantyhose, in which polyurethaneurea elastic fiber is often used, an improvement in the transparency of a polyurethaneurea elastic fiber is required. To satisfy this requirement, production of fine denier fiber is required. Moreover, in order to improve the productivity, in relation to a converting process, high-draft processing and high-speed processing are required. To satisfy the above-mentioned requirements, it is necessary that the breaking strength, preferably the breaking strength together with the elongation at break, of a polyurethaneurea elastic fiber should be increased.
A polyurethaneurea solution used in the manufacture of polyurethaneurea elastic fiber tends to give rise to occurrences of partial gel (or gelation) and abrupt and intense increase in viscosity due to the cohesion of hard segments in the polymer whereby the shaping process becomes unstable. Polyurethaneurea elastic fiber prepared from such solution cannot exhibit either a high breaking strength or a high elongation at break.
There have been several attempts to improve polyurethaneurea fiber in its breaking strength and its elongation at break. In connection with a technology for removing the instability of a spinning dope, which is caused by the cohesion of hard segments in the polyurethaneurea solution, there are known, for instance, Japanese Examined Patent Publication (Kokoku) No.44-22113 and No. 45-10956.
Japanese Examined Patent Publication (Kokoku) No. 44-22113 describes an improvement in spinning dope in which the stability of polyurethaneurea solution is increased by reacting an intermediate polymer (prepolymer) with isocyanate terminal groups at both ends with a small amount of monofunctional alcohol and successively reacting the mixture with a bifunctional amine to form a chain extended polymer, in order to improve stability of the polyurethaneurea solution and the spinnability of the spinning dope. With an improvement in spinnability, the resultant elastic fiber is improved in its breaking strength and elongation at break. However, it is to be noted that, since the breaking strength described in the publication is 1 g/d at most, the improvement in the strength is not large. In the description of the publication, there is stated, "It is well known that a compound such as a metal salt, an alkali, an amine or the like prompts a formation of gelated reaction product in a urethane reaction." This statement discloses the necessity of the removal of these compounds.
Further, Japanese Examined Patent Publication (Kokoku) No. 45-10956 describes that objectives pertaining mainly to the prevention of gel forming and the stabilization of a polyurethaneurea solution are achieved by a method for chain extension of a prepolymer solution in which a prescribed amount of a previously added monofunctional amine is reacted before the chain extension with the addition of a bifunctional amine is carried out. Further, the method improves the strength of the fiber by promoting suitable formation of crosslinking or branching by side-reaction other than the chain extension reaction at the time when a monofunctional amine is reacted. The resultant dope is stable; however, the fiber is not necessarily satisfactory because the resultant fiber exhibits a strength of 1.28 g/d and an elongation of 580%. It is disclosed that the improvement effect on tensile properties seems to be relatively great in contrast to the comparative gelated dope. However in general, a fiber prepared from a dope involving a side-reaction exhibits an improved strength and modulus, however the elongation is rather deteriorated.
A polyurethaneurea elastic fiber obtained by the above known technology, cannot reach either the required breaking strength or the required elongation at break.
Japanese Unexamined Patent Publication (Kokai) No. 1-166426 describes a technology for improving an antistatic property of a polyurethaneurea elastic fiber in the course of converting by incorporating a metal salt of sulfonic acid therein where propylenediamine (1,2-diaminopropane) is solely used as a bifunctional amine. The known disclosure does not suggest the effect of the sulfonate additive on the material properties of polyurethaneurea elastic fiber such as strength, elongation, and the like.
According to a knowledge attained by the present inventors, a polyurethaneurea making use of 1,2-diaminopropane as a bifunctional amine rather loses its strength when the sulfonate is added.